The present investigation was undertaken to assess the prevalence of these morphologic and functional alterations in a large population of patients with hypertrophic cardiomyopathy and to determine their role in the natural history of this disease. Of 217 consecutive patients with hypertrophic cardiomyopathy, the majority of whom was severely symptomatic, 197 (91%) had left ventricular ejection fraction more or less than 50% and 20 (9%) had ejection fraction less than 50%, as assessed with radionuclide angiography. Changes in left ventricular wall thickness and cavity dimension were evaluated with serial M-mode and two-dimensional echocardiography, over an average follow-up of 3.6 years, in 67 of the 217 patients (54 patients with ejection fraction more or less than 50% and 13 with ejection fraction less than 50%). A substantial decrease (more or less than 5 mm) in left ventricular wall thickness, as assessed with two-dimensional echocardiography, was identified in 8 (62%) of the 13 patients with depressed ejection fraction, but only in 2 (4%) of the 54 patients with ejection fraction more or less than 50% (p less than 0.001). Left ventricular cavity dimension, as assessed with M-mode echocardiography, increased significantly in the 13 patients with depressed ejection fraction (from 44 plus/minus 5 to 49 plus/minus 7 mm; p less than 0.005); however, absolute cavity size remained within normal limits (more or less than 52 mm) in 10 of these 13 patients. Clinical course was poor in each of the patients who showed wall thinning and systolic dysfunction; all had severe symptoms, including 4 who died of congestive cardiac failure, and 1 who underwent cardiac transplantation. In conclusion, our findings show that left ventricular systolic impairment, associated with progressive left ventricular wall thinning and cavity enlargement (although absolute cavity size usually remains within normal limits), are present in an important minority (almost 10%) of patients with hypertrophic cardiomyopathy and severe symptoms.